Tesla coil


A Tesla coil is an electrical resonant transformer device designed by inventor Nikola Tesla in 1891. It is used to produce high voltage, low current, high frequency alternating current. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits.
Tesla used these devices to conduct innovative experiments in electrical lighting, phosphorescence, X-ray generation, high-frequency alternating current phenomena, electrotherapy, and the wireless transmission of electrical energy. Tesla coil circuits were used commercially in spark-gap transmitters for wireless telegraphy until the 1920s, and in medical equipment such as electrotherapy and violet ray devices. Today, their main usage is for entertainment and educational displays, although small coils are still used as leak detectors for high-vacuum systems.
Originally, Tesla coils used fixed spark gaps or rotary spark gaps to provide intermittent excitation of the resonant circuit; more recently, electronic devices are used to provide the switching action required.

Operation

A Tesla coil is a high-frequency, air-cored resonant transformer, which can be used to produce very high voltages. In Tesla’s early designs, a simple spark gap was used to set up the oscillations in a tuned, radio-frequency transformer.
Tesla coils can produce output voltages from 50 kilovolts to several million volts for large coils. The alternating current output is in the low radio frequency range, usually between 50 kHz and 1 MHz.
The original, spark-excited Tesla coil circuit, shown below, consists of these components:
  • A high-voltage supply transformer ', to step the AC mains voltage up to a high enough voltage to jump the spark gap. Typical voltages are between 5 and 30 kilovolts.
  • A capacitor ' that forms a tuned circuit with the primary winding L1 of the Tesla transformer
  • A spark gap ' that acts as a switch in the primary circuit
  • The Tesla coil ', an air-core double-tuned resonant transformer, that generates the high output voltage.
  • Optionally, a capacitive electrode in the form of a smooth metal sphere or torus attached to the secondary terminal of the coil. Its large surface area suppresses premature air breakdown and arc discharges, increasing the Q factor and output voltage.

    Resonant transformer

The specialized transformer used in the Tesla coil circuit ', called a resonant transformer or radio-frequency transformer, functions on different principles to transformers used in AC power circuits. While an iron-cored transformer is designed to transfer energy efficiently from primary to secondary winding, the resonant transformer is designed to temporarily store and transfer high frequency currents. A primary winding is in parallel with a capacitor, while the secondary winding has a "parasitic' capacitance with its inductor, as illustrated by the diagram. The inductors and capacitors together function as two LC circuits, "tuned" so they have the same resonant frequency, and each spark briefly completes a circuit on the primary side, producing oscillation back and forth between the primary and secondary. This is analogous to the way a tuning fork stores vibrational mechanical energy.
The primary coil ' consists of relatively few turns of heavy copper wire or tubing, and is wired in parallel to a capacitor '. Current flows through the spark gap with each spark, and repetitively energizes the primary side LC circuit. The secondary coil ' consists of hundreds to thousands of turns of wire on a hollow cylindrical form, and the primary is coupled to it by being wound on an insulating former outside it.
The inductance of ' resonates with stray capacitance ' and the capacitance of the toroid's metal electrode attached to the high-voltage terminal.
The peculiar design of the coil is dictated by the need to achieve low resistive energy losses at high frequencies, which results in the largest secondary voltages:
  • The coils in a Tesla transformer are loosely coupled. The primary winding is larger in diameter and spaced apart from the secondary, so the mutual inductance is low and the coupling coefficient is typically 0.05 to 0.2. The loose coupling slows the exchange of energy between the primary and secondary coils, which allows the oscillating energy to stay in the secondary circuit longer before it returns to the primary and begins dissipating in the spark.
The output circuit can have two forms:
  • Unipolar: One end of the secondary winding is connected to a single high-voltage terminal, the other end is grounded. This type is used in modern coils designed for entertainment. The primary winding is located near the bottom, low potential end of the secondary, to minimize arcs between the windings. Since the ground serves as the return path for the high voltage, streamer arcs from the terminal tend to jump to any nearby grounded object.
  • Bipolar: Neither end of the secondary winding is grounded, and both are brought out to high-voltage terminals. The primary winding is located at the center of the secondary coil, equidistant between the two high potential terminals, to discourage arcing.

    Operation cycle

The circuit operates in a rapidly repeating cycle in which the supply transformer ' charges the primary capacitor ' up, which then discharges in a spark through the spark gap, creating a brief pulse of oscillating current in the primary circuit which excites a high oscillating voltage across the secondary:
  1. Current from the supply transformer ' charges the capacitor ' to a high voltage.
  2. When the voltage across the capacitor reaches the breakdown voltage of the spark gap ' a spark starts, reducing the spark gap resistance to a very low value. This completes the primary circuit and current from the capacitor flows through the primary coil '. The current flows rapidly back and forth between the plates of the capacitor through the coil, generating radio frequency oscillating current in the primary circuit at the circuit's resonant frequency.
  3. The oscillating magnetic field of the primary winding induces an oscillating current in the secondary winding ', by Faraday's law of induction. Over a number of cycles, the energy in the primary circuit is transferred to the secondary. The total energy in the tuned circuits is limited to the energy originally stored in the capacitor C1, so as the oscillating voltage in the secondary increases in amplitude the oscillations in the primary decrease to zero. Although the ends of the secondary coil are open, it also acts as a tuned circuit due to the capacitance ', the sum of the parasitic capacitance between the turns of the coil plus the capacitance of the toroid electrode E. Current flows rapidly back and forth through the secondary coil between its ends. Because of the small capacitance, the oscillating voltage across the secondary coil which appears on the output terminal is much larger than the primary voltage.
  4. The secondary current creates a magnetic field that induces voltage back in the primary coil, and over a number of additional cycles the energy is transferred back to the primary, causing the oscillating voltage in the secondary to decrease. This process repeats, the energy shifting rapidly back and forth between the primary and secondary tuned circuits. The oscillating currents in the primary and secondary gradually die out due to energy dissipated as heat in the spark gap and resistance of the coil.
  5. When the current through the spark gap is no longer sufficient to keep the air in the gap ionized, the spark stops, terminating the current in the primary circuit. The oscillating current in the secondary may continue for some time.
  6. The current from the supply transformer begins charging the capacitor C1 again and the cycle repeats.
This entire cycle takes place very rapidly, the oscillations dying out in a time of the order of a millisecond. Each spark across the spark gap produces a pulse of damped sinusoidal high voltage at the output terminal of the coil. Each pulse dies out before the next spark occurs, so the coil generates a string of damped waves, not a continuous sinusoidal voltage. The high voltage from the supply transformer that charges the capacitor is a 50 or 60 Hz sine wave. Depending on how the spark gap is set, usually one or two sparks occur at the peak of each half-cycle of the mains current, so there are more than a hundred sparks per second. Thus the spark at the spark gap appears continuous, as do the high-voltage streamers from the top of the coil.
The supply transformer secondary winding is connected across the primary tuned circuit. It might seem that the transformer would be a leakage path for the RF current, damping the oscillations. However its large inductance gives it a very high impedance at the resonant frequency, so it acts as an open circuit to the oscillating current. If the supply transformer has inadequate short-circuit inductance, radio frequency chokes are placed in its secondary leads to block the RF current.

Oscillation frequency

To produce the largest output voltage, the primary and secondary tuned circuits are adjusted to resonance with each other. The resonant frequencies of the primary and secondary circuits, and, are determined by the inductance and capacitance in each circuit:
Generally the secondary is not adjustable, so the primary circuit is tuned, usually by a moveable tap on the primary coil L1, until it resonates at the same frequency as the secondary:
Thus the condition for resonance between primary and secondary is:
The resonant frequency of Tesla coils is in the low radio frequency range, usually between 50 kHz and 1 MHz. However, because of the impulsive nature of the spark they produce broadband radio noise, and without shielding can be a significant source of RFI, interfering with nearby radio and television reception.